An optical transmission apparatus based on duobinary signaling is known, for example, as shown in “The duobinary technique for high-speed data transmission, IEEE Transaction on Communication & Electronics, Vol. 82, 1963” and “Characteristics of Optical Duobinary Signals in Terabit/s Capacity, High-Spectral Efficiency WDM Systems, IEEE Journal of Lightwave Technology, Vol. 16, No. 5, 1998”.
Thus, the duobinary signaling had been studied in the 1960's, and it is the basis of the partial response signaling employed in the radio baseband transmission system. The duobinary signaling has also been employed for narrowing optical spectra for optical modulation since the early 1990's. In the duobinary signaling, a binary signal of [0/1] is converted to a ternary signal of [0/1/2], and therefore a frequency bandwidth is compressed. A decoder at a recipient converts a [0, 2] signal to a [0] signal and converts a [1] signal to a [1] signal, and therefore the binary signal of [0/1] generated at a sender is decoded.
FIG. 18 is a block diagram of an optical transmission apparatus using the conventional duobinary signaling. In FIG. 18, N-parallel signals S201 with a low transmission rate of B/N [bit/sec] is multiplexed into a serial binary signal S202 with a transmission rate of B [bit/sec] by a multiplexing circuit 201. This binary signal S202 is transmitted to a precoder 202. The precoder 202 applies a processing for reducing intersymbol interference between bits to the binary signal S202, and thus outputs a binary signal S203. The binary signal S203 is transmitted to an encoder 203 that converts the binary signal S203 to a ternary signal S204.
The ternary signal S204, by an optical modulator 204, is converted to an optical ternary signal in which the optical electric field strength (phase) has a ternary value of [1(0), 1(π), 0(no phase)]. In an optical-to-electrical (O/E) converter 206a included in a decoder 206 at a recipient, the optical ternary signal is converted to an electric current signal depending on the light intensity of the optical ternary signal by a photodetector. As a result, the phase information of the optical ternary signal is lost, and the optical ternary signal is converted to a binary electric signal S206. This binary electric signal S206 corresponds to the binary signal S202 generated at a sender.
The combination of the precoder 202 and the encoder 203 makes it possible to convert, with respect to the binary signal S202, the [0] value to [0],value or [2] value, and the [1] value to [1] value. The precoder 202 has an EXOR gate 202a and a delay circuit 202b. The delay circuit 202b delays an inverting output of the EXOR gate 202a by T (=1/B) [sec], and transmits the output delayed to the EXOR gate 202a. 
The encoder 203 includes a delay circuit 203a and an adder 203b. The binary signal S203 is split into two signals in the encoder 203. One of the signals, by the delay circuit 203a, is delayed by a delay time difference T [sec]. The signal delayed and the other signal are added by the adder 203b in an analog, and therefore the adder 203b outputs a ternary signal S204 as the addition result.
Specifically, the encoder 203 can be realized by a configuration shown in FIG. 19. An encoder 213 shown in FIG. 19(a) includes a flip-flop circuit 213a and a low-pass filter 213b. The low-pass filter 213b is connected to the flip-flop circuit 213a in a subsequent stage, and has a cutoff frequency of B/4 [Hz]. An encoder 223 shown in FIG. 19(b) includes an adder 223c and a shift register that is the combination of two flip-flop circuits 223a and 223b. The adder 223c adds two signals output from the flip-flop circuits 223a and 223b. 
The encoders 213 and 223 shown in FIG. 19(a) and FIG. 19(b) have the same function. That is, the encoders 213 and 223 generates an output signal from an input signal at a certain timing, in which the input signal is thinned out by one clock and extended by one clock, between two clocks. For example, a signal of “1, 1” is generated from an input signal “1”, and a signal of “0, 0” is generated from an input signal “0”. The encoder 223 adds these signals S201 and S202 by the adder 223c, and outputs the addition result as a ternary signal S204, as shown in FIG. 20.
In the conventional optical transmission apparatus, the output signal of the precoder 202 and the output signal of the shift register in the encoder 223 change at B [bit/sec] same as the transmission rate B of the binary signal S202, and must be processed at this rate.
Therefore, there is a problem in that an electronic device, for example, a high-speed flip-flop circuit, which can operate at the same transmission rate B [bit/sec] as that of the multiplexed binary signal S202, is essential.
Particularly, in the optical transmission path constituting a backbone network, it is advantageous in view of cost to make the optical transmission rate as high as possible, but the operation speed of the electronic device such as the flip-flop circuit limits the optical transmission rate. Therefore, it is desired to obtain high-speed optical transmission from an electronic device having a speed as low as possible.
It is therefore an object of the present invention to provide a multiplexer that can perform high-speed optical transmission without using the electronic device such as the flip-flop circuit operating at a high speed, enables optical transmission rate exceeding the operation speed limit of the normal flip-flop circuit, and can contribute to low cost and miniaturization.